Linear low density polyethylene (LLDFE) is an ethylene polymer prepared using a coordination catalyst in the same manner used in preparing linear high density polyethylene (HDFE), and is actually a copolymer of ethylene and at least one higher alpha-olefin. The expression "linear ethylene polymers" includes those linear ethylene polymers which have from 0 percent to about 30 percent of at least one higher alpha-olefin of 3 to 12 carbon atoms copolymerized with the ethylene.
According to Modern Plastics Encyclopedia, linear high density polyethylene generally has a density in the range of about 0.941 gm/cm.sup.3 to about 0.965 gm/cm.sup.3. Skilled artisans realize that the density can vary in accordance with the reaction conditions and type of catalyst used. It is also known that densities a little above that range can be achieved by special techniques, such as by annealing the polymer. It is known that polymerization conditions which will produce a homopolymer having a density of about 0.965 gm/cm.sup.3 will produce a copolymer of ethylene and higher alphaolefin having a density lower than the said 0.965 gm/cm.sup.3, the extent of the lowering of the density being directly related with the amount of the higher alphaolefin used.
It is known from EFO 85 101380.5 (U.S. Pat. No. 4,830,907) that linear low density polyethylene (LLDFE) can be made into fibers by melt-spinning and that even very fine fibers can be produced.
lt is known from U.S. Pat. No. 4,578,414 that wettable fibers of polyethylene, including LLDFE, can be prepared from polyethylene having incorporated therein certain wetting agents.
The manufacture of LLDFE is disclosed. e.g., in U.S. Pat. No. 4,076,698, which discloses that the LLDFE can be extruded as a monofilament and then cold drawn. It is shown as being a copolymer of ethylene and at least one other higher alpha-olefin. The density of the LLDPE is dependent on the amount of the higher alphaolefin in the copolymer.
Whereas LLDPE has been introduced in the market place as a polymer suitable for making fibers, it is known that not all versions and varieties of LLDFE are entirely adequate for commercial production of staple fibers and the fabric strength of carded staple-based fabrics has generally been significantly less than fabrics made from carded staple-based polypropylene fibers.
lt is recognized in the art of making LLDPE polymers that the density of the LLDPE is affected by the amount and kind of olefin comonomer which is copolymerized with the ethylene and, to some extent, by the process conditions and catalyst used. A given mole percent of, e.g., propylene in the copolymer will reduce the density of the polyethylene less than the same mole percent of a higher olefin comonomer. The MFR (melt flow rate as measured by ASTM D-1238) is also affected to some degree by the kind and amount of olefin comonomer in the copolymer, and is also affected to some extent by the coordination catalyst used, the polymerization conditions, and/or by any telogens or chain regulators or other reactants which may be present during polymerization.
It is also recognized in the art that there are important distinctions between linear polyethylenes (which includes LLDPE polymers), and branched-chain ethylene polymers, which are made using a free-radical catalyst and are generally referred to as LDPE (low density polyethylene), and were also known in the past as ICI-type polyethlyene and as HPFE (high pressure polyethylene). This disclosure deals with linear polyethylenes.